VFTO Induced TGPR Analysis for 400kV GIS Substation

Transient enclosure voltage (TEV) also known as Transient ground potential rise (TGPR) is a special case of VFTO. This phenomenon refers to short rise time, short duration high voltage transient which appear on earthed enclosure of GIS. This is mainly through the coupling of internal transients on conductor refracted to the enclosure at enclosure discontinuities such as air terminations, insulated flanges at GIS/Cable interfaces and some current transformers. Primarily disconnector switch operations are causes of VFTO and SF6-Air bushing seems to be the most significant source of TEV. Observation of sparking between grounded enclosure and support structure, failure of protective devices, inadvertent operation of relays etc are common malfunction of TEV. Despite proper grounding this phenomenon indicates presence of high potentials on GIS enclosures which raises the issue of equipment protection, migration of these transients to adjacent equipment and shock hazards. TEV in 400kV GIS substation due to “disconnector switch operation” was studied and simulations performed for GIS model with the equivalent resistance of ground grid, resistance and inductance of grounding strip using EMTP. The presentation summarizes the results of study needed for a 400kV GIS substation in India to ascertain VFTO induced TGPR level. The study was carried out by modeling all GIS equipment, enclosure in EMTP and careful consideration of grounding system model. The results of the study were analyzed to decide on the requirement of mitigation methods to ensure the permissible level of TEV.